1. Field of the Invention
The present invention relates to a projection-type image display apparatus for use in a projection-type high-definition television system or video projector. More particularly, the present invention relates to an image display apparatus having a unique feature in the structure of the optical system for projecting a color image.
2. Description of the Related Art
One of known conventional projection-type image display apparatus is a projection-type color liquid-crystal display apparatus using a liquid-crystal display panel.
The projection-type color liquid-crystal display apparatuses are roughly divided into a three-panel color liquid-crystal display apparatus having three liquid-crystal display panels for red, green and blue primaries and a single-panel color liquid-crystal display apparatus having a single liquid-crystal display panel with a mosaic- or stripe-patterned tricolor filter.
The three-panel color liquid-crystal display apparatus comprises an optical system for converting a white light into red, green and blue primaries, and a liquid-crystal display panel for controlling luminance of each color to form an image.
The final color image is produced and presented by optically superimposing red, green, and blue color images.
The single-panel color liquid-crystal display apparatus introduces a white light into a single liquid-crystal display panel with a mosaic- or stripe-patterned tricolor filter to form and present a color image.
Besides the three-panel and single-panel projection-type liquid-crystal display apparatuses, another projection-type color image display apparatus using a digital micromirror device (DMD: trade name of Texas Instruments) is recently available (reference is made to magazine "Optics", vol. 25, No. 6, p.313-314, 1996).
The liquid-crystal panel used in the three-panel or single-panel projection-type liquid-crystal display apparatus, as already known, controls a number of two-dimensionally arrayed cells of liquid-crystal molecules in orientation to vary polarization of light, thereby switching on and off the transmission of light.
The above-cited DMD, having a two-dimensional array of a number of pixels each having a micromirror, controls the tilt of each mirror individually through the effect of electrostatic field caused by a memory element arranged respectively for each pixel and varies the angle of reflection of reflected light ray thereby causing on/off state.
FIG. 9 shows the operation of the micromirror arranged for each pixel in DMD. Diagrammatically shown in FIG. 9 are micromirrors 101 through 105 and a projection lens 110. As shown, the pixels corresponding to micromirrors 103 and 105 are in the on state.
Light rays reflected off the micromirrors 101, 102, and 104 of pixels in the off state are not directed to the projection lens 110. Light rays reflected off the micromirrors 103 and 105 of the pixels in the on state are directed to the projection lens 110 and forms an image on a screen.
The tilt angle of the micromirror of a pixel in the on state is 10 degrees or so with respect to a horizontally aligned micromirror.
The advantages of DMD over the liquid-crystal display panel employing a polarizer include a better utilization of light, heat resistance property, high-speed response characteristics, and the like.
FIG. 10 is a perspective view of an optical system for a conventional projection-type color image display apparatus using a DMD.
A white light arc lamp (light emission point) 51 such as a xenon arc lamp is arranged at one focus of a collector ellipsoidal mirror 52.
The light beam emitted from the arc lamp 51 is focused at the other focus of the ellipsoidal mirror 52, thereby forming a virtual secondary light source.
A rotatable color filter 53 is placed at the position of the secondary light source (the other focus of the ellipsoidal mirror 52).
As shown in FIG. 11, the color filter 53 has a ring portion which is partitioned into transmission-type filters 53R, 53G and 53B correspondingly to three primaries of red, green and blue. Designated 531 is the axis of rotation of the color filter 53.
When rotated about the axis of rotation 531 in parallel with the optical axis of light beam from the arc lamp 51 shown in FIG. 10, the color filter 53 converts sequentially the white light into red, green and blue colors.
Referring to FIG. 10, a light beam passing through the color filter 53 is transmitted through condenser lenses 541, 542, reflected from a plane mirror 551, and transmitted through a condenser lens 543. The light beam transmitted through the condenser lens 543 is reflected from a plane mirror 552, transmitted through a condenser lens 544, and introduced into DMD 56. The light beam reflected from DMD 56 is admitted to a projection lens 57.
The condenser lenses 541-544 have a function of condensing red, green or blue light beam at an entrance pupil of the projection lens 57 through the micromirrors of pixels in the on state of DMD 56. Furthermore, these condenser lenses 541-544 have a function of reducing nonuniform illumination caused by uneven illuminance on the screen.
The plane mirrors 551 and 552 have a function of bending, in a three-dimensional space, the optical path of an optical illumination module that is complicatedly routed through the condenser lenses 541-544. The optical illumination module refers to an optical system constituted by components present in the optical path extending along the light beam from the arc lamp 51 to DMD 56.
The reason the optical path of the optical illumination module is made complicated is as follows. To make DMD 56 to work correctly, the angle of incidence of a light beam to the surface of each micromirror in DMD 56 is necessarily great (80 degrees, for example), and as a result, the components constituting the optical illumination module, such as the condenser lenses, are subject to mechanical contact with or interference with the projection lens 57.
To preclude mechanical contact or interference, the plane mirrors 551 and 552 are necessarily three-dimensionally placed, as shown in FIG. 10, thereby making the optical path of the optical illumination module complex.
The central axis of DMD 56 is not colinearly aligned with the optical axis of the projection lens 57, and DMD 56 is offset (shifted) from the optical axis of the projection lens 57. In the conventional art, the projection lens 57 is therefore used partially rather than in its full angle of view.
Because of its complex structure, the three-panel projection-type color image display apparatus is bulky and costly.
Since the single-panel projection-type color display apparatus features a relatively simple optical structure and small component count, compact and low-cost design is easily implemented. On the other hand, the use of a color filter presents difficulty in full utilization of light beams from the light source and results in a darker image. If luminance of the light source is raised to compensate for this disadvantage, components such as the liquidcrystal panel must be provided with sufficient cooling steps.
The conventional projection-type color image display apparatus using DMD is particularly heat-resistant, and presents a high resolution because of a fine grid as compared with the liquid-crystal display panel. However, the DMD color image display apparatus has the following disadvantages.
As is apparent from FIG. 10, the component count of the optical illumination module is so large that a potential high luminance image advantage of DMD cannot be fully exploited.
Furthermore, since the optical illumination module is arranged three-dimensionally, the assembly and adjustment process is time-consuming and the apparatus becomes bulky and costly.
It is an object of the present invention to provide an image display apparatus that presents a high-luminance and high-illuminance color image through high utilization of a light beam.
It is another object of the present invention to provide an image display apparatus that implements compact and low-cost design by reducing a component count of an optical illumination module.